Wavelength selective switches (WSS) are widely used for dynamic routing wavelength channels in optical communications networks. WSS devices are deployed in optical switching nodes of long-haul, regional, and metro optical communications networks. Referring to FIG. 1A, a typical WSS 100 includes a front end 101 having a single fiber array unit (FAU) 102, containing, for example, one input and four output optical fibers, a concave, usually spherical, mirror 104 to expand and steer an input optical beam 105 emitted by the input fiber of the FAU 102 onto a diffraction grating 106 for wavelength dispersion, further imaging onto a dispersed focal plane 108 containing a MEMS array 110 for selectively steering wavelength channels back through the WSS 100 to the outputs fibers of the FAU 102. Such wavelength-selective switching devices have been disclosed, for example, by Bouevitch et al. in U.S. Pat. No. 6,498,872, and by Ducellier et al. in U.S. Pat. No. 6,707,959. The MEMS array 110 can be replaced by a digital light processor (DLP), liquid crystal (LC), or a liquid-crystal-on-silicon (LCoS) phased array. In the latter two cases, due to polarization sensitivity of the MEMS array 110, the optical beam 105 is split into two orthogonally polarized sub-beams at the front end 101, with one sub-beam being rotated so that the optical sub-beams in a single polarization state are propagated through the WSS 100.
Presently available WSS are highly integrated devices. Keyworth et al. in U.S. Pat. No. 7,725,027 disclose a twin WSS device, in which two WSS share common optical elements. Referring to FIGS. 1B and 1C with further reference to FIG. 1A, a prior-art twin WSS 120 is shown in schematic elevation and plan views, respectively. The twin WSS 120 includes two independently operable WSS units sharing the following common optical elements: a collimator 104-1, the diffraction grating 106, a focusing element 104-2, and the MEMS 110. The collimator 104-1 and the focusing element 104-2 can be replaced by the single spherical mirror 104 of FIG. 1A, resulting in a practical 4f configuration. The 4f configuration for a WSS has been described in detail in the U.S. Pat. No. 6,498,872.
Referring specifically to FIG. 1B, a first WSS unit includes a first front end 101A having a first FAU 102A and a first lens 121A. A light beam 105A is collimated by the collimator 104-1, dispersed into individual wavelength channels 107A (FIG. 1C) by the diffraction grating 106, and is focused by the focusing element 104-2 onto a first row 110A of tiltable micromirrors of the MEMS array 110, which redirect individual wavelength channels 107A to propagate back and couple in a wavelength-selective manner into output fibers of the first FAU 102A. Similarly, a second WSS unit includes a second front end 101B having a second FAU 102B and a second lens 121B. A light beam 105B is collimated by the collimator 104-1, dispersed into individual wavelength channels 107B (FIG. 1C) by the diffraction grating 106, and is focused the focusing element 104-2 onto a second row 110B of tiltable micromirrors of the MEMS 110, which redirect individual wavelength channels 107B to propagate back and couple in a wavelength-selective manner into output fibers of the second FAU 102B. In the view of FIG. 1C, the wavelength channels 107A and 107B are overlapped.
The twin WSS 120 of FIGS. 1B and 1C is a more economical and space-efficient unit than two single-unit WSS 100 of FIG. 1A. Future networks will require higher-degree network nodes having higher port count, and “colorless” add/drop ports, that is, add/drop ports allowing any wavelength channel to be added or dropped. The twin WSS 120 operates over a single spectral band of communication, either C-band or L-band.
Increasing data throughput in WSS networks has typically been achieved through a combination of increased number of multiplexed wavelengths within the C or L-band, and increased bit rates within each wavelength channel. When combined, these increases lead to improved spectral efficiency within the finite optical bandwidth of the optical amplifiers. However, each subsequent increase requires an improved optical signal-to-noise ratio (OSNR), which typically requires that optical power levels in the fiber are increased. It is anticipated that within the next 5 years, one will reach fundamental limits due to fiber optical nonlinearity, preventing further increases in OSNR. At this point, it is likely that the next path forward would either be C+L band WSS networks, or “spatially multiplexed” networks, consisting of overlayed “planes” of WSS networks. Demand is expected for both solutions, depending on the existing infrastructure of network operators; to support this future demand of compact, economical WSS devices, a more densely packed WSS devices are required.